The present invention pertains to an apparatus and method for manufacturing bioprosthetic heart valves and, more particularly, to a sizer and method of sizing fresh donor heart valves to facilitate the fabrication of bioprosthetic heart valves.
Prosthetic heart valves are used to replace damaged or diseased heart valves. In vertebrate animals, the heart is a hollow muscular organ having four pumping chambers: the left and right atria and the left and right ventricles, each provided with its own one-way valve. The natural heart valves are identified as the aortic, mitral (or bicuspid), tricuspid and pulmonary valves. Prosthetic heart valves can be used to replace any of these naturally occurring valves, although repair or replacement of the aortic or mitral valves is most common because they reside in the left side of the heart where pressures are the greatest.
Where replacement of a heart valve is indicated, the dysfunctional valve is typically cut out and replaced with either a mechanical valve, or a tissue valve. Tissue (e.g., xenograft) valves are often preferred over mechanical valves because they typically do not require long-term treatment with anticoagulants. Although so-called stentless valves are available, the most widely used tissue valves include some form of stent or synthetic leaflet support. The most common tissue valves are constructed with an intact, multi-leaflet donor valve, or with separate leaflets cut from bovine (cow) pericardium, for example. The most common intact valve used for stented and stentless valves is the porcine (pig) aortic valve, although other porcine valves and valves from other animals (e.g., equine or marsupial donors) have been used. The present invention is not limited to the preparation of porcine valves, though existing bioprosthetic heart valves on the market are nearly exclusively made from porcine valves, and thus the description herein will focus on such valves.
In a typical prosthetic valve fabrication process, the fresh porcine heart is first harvested in a certified slaughterhouse from the animal, weighed, and sorted into various valve size ranges by means of either estimating sized by eye based on the flattened aortic width, or by heart weight to valve size correlation. Of course, this correlation is a very rough estimate, with actual valve sizes differing quite a bit within similarly-sized porcine hearts. The aortic valve and surrounding tissue (hereinafter termed the xe2x80x9caortic valve isolationxe2x80x9d) is then severed from the porcine heart. Because of its proximity to the aortic valve, the pulmonary artery remains connected to the aortic valve isolation. A cross-section of the aortic valve isolation can be seen in FIG. 4 in the context of the sizer and method of sizing of the present invention.
At this stage, a large number of aortic valve isolations are packed in ice and shipped from the slaughterhouse to the prosthetic valve manufacturing facility. At the manufacturing facility, the aortic valve isolation is further sorted by valve size by technicians trained to estimate such valve size using their fingers. That is, the orifice diameter of the aortic valve annulus is estimated by insertion of one or more fingers through the inflow end of the aortic valve isolation. Because of the rough nature of the heart weight to valve size estimation, a large proportion of valves are rejected at this stage, resulting in wasted inventory and shipping costs.
It should be noted that the aortic valve annulus defines the narrowest opening through the valve, and is the reference dimension for implantation purposes. That is, the annulus diameter of the human patient is measured using conventional surgical sizers to determine the orifice size of the replacement bioprosthetic valve. Conventional sizers for measuring the human valve annulus typically comprise a series of incrementally-sized cylindrical elements marked with the corresponding outside diameter in mm. Most sizer sets include cylindrical elements that range from a low of 19 mm to a high of 33 mm, in 2 mm increments, and a common handle for manipulating the sizers. Some sizers for measuring the human valve annulus are shaped, or include flanges or other stepped features to also provide a measurement of the aortic root adjacent to the annulus. The aortic root is that part of the valve anatomy between the annulus and the convex sinuses of the ascending aorta, and has a generally scalloped appearance with the valve leaflets being attached along alternating arcuate cusps and upstanding commissures around its border. In any event, the primary measurement derived from conventional surgical sizers is the annulus diameter determined by finding which sizer fits properly in the annulus based on tactile feedback.
Following the estimation of the porcine aortic valve annulus diameter by the finger measurement technique, the fresh valve is then trimmed and chemically fixed to render it biologically inert for implantation purposes. The trimming procedure typically involves cutting away the pulmonary artery and surrounding muscle tissue from the inflow end of the valve. What is left is a generally tubular valve element having a small amount of tissue on the inflow side of the annulus, with the internal leaflets being enclosed and protected by the tubular ascending aorta. Chemical fixation may be accomplished using a variety of techniques and chemicals, though the most common procedure used involves supporting the tubular valve element on at least the ascending aorta or outflow portion with a fixation insert, immersing the assembly in a bath of fixing solution (e.g., glutaraldehyde), and either flowing fixing solution through the valve element or maintaining a predetermined pressure differential across the leaflets during the fixation process. See, for example, U.S. Pat. No. 4,372,743 to Lane, which describes maintaining a low pressure differential across the leaflets of between 1-4 mm Hg.
The use of fixation inserts is also quite effective in shaping the valve during the fixation process. For example, U.S. Pat. No. 5,197,979 to Quintero, et al. describes inserts having three outwardly convex regions for shaping the valve sinuses. More recently, U.S. Pat. No. 6,001,126 to Nguyen, et al. discloses inserts having a plurality of pin holes in the two convex regions corresponding to the coronary sinuses that enable coronary artery shaping plugs or mandrels to be mounted thereon. Whichever type of insert is used, the ultimate size of the fixed valve is influenced, at least in the sinus regions, by the insert. Preferably, the relative size of the annulus and sinus regions is identical to the human aortic valve being replaced. It is therefore very important to begin with a donor valve having an accurately sized annulus.
The fixation process causes some shrinkage in the tissue. Therefore, the sizing of fresh tissue provides only an estimate of the annulus size of the fixed tissue. The amount of shrinkage depends on the chemicals used, the duration of fixation, the pressure differentials within the valve, any heating that is applied, and other less significant factors. Because of these variables, fixed porcine aortic valves are sized once again using a caliper and/or a sizing stent to sort the valves into mounting sizes.
Another consideration for proper valve sizing is the dynamic expansion and contraction experienced in use after implantation. One study by Hansen, entitled Longitudinal and Radial Distensibility of the Porcine Aortic Root (Department of Electrical Engineering, the University of Western Ontario, London, Ontario, June 1994) showed that the aortic root might contract radially up to 25%, and longitudinally up to 12% when heart is arrested and the aortic root is under no pressure. The study suggests sizing the bioprosthetic replacement valve approximately 30% greater in diameter than the native aortic root at zero pressure to accommodate the expected expansion.
It is thus apparent that an accurate and reliable means for estimating, from the fresh valve, the annulus size of a fixed xenograft valve annulus is needed to increase valve yield and quality, and reduce expense.
The present invention provides an apparatus for sizing fresh donor heart valves that have a lumen and an inwardly-directed valve annulus within the lumen. The apparatus includes a sizing member having an axially-extending sizing portion with a forward end adapted to insert within the lumen of the donor heart valve. The sizing portion increases in size along an axis from the forward end such that a region on the exterior thereof eventually contacts the valve annulus upon continued insertion of the sizing portion within the lumen. A measuring bracket connects to the sizing member and has a scale portion spaced from and generally aligned with the sizing portion, the scale portion providing markings indicating the annulus size of the donor heart valve relative to the position of the donor heart valve on the sizing portion. In a preferred embodiment, the sizing portion is conical having a taper of between 1-6 degrees. The measuring bracket may include a mounting portion generally perpendicular to the scale portion and including a through hole into which the sizing portion fits in an interference.
In a further embodiment, the present invention provides a method of measuring the annulus of a fresh donor heart valve including the steps of obtaining a fresh donor heart valve, and providing a sizer having an axially-extending sizing portion adapted to fit within a lumen of the fresh donor heart valve. The sizing portion has an exterior surface that increases in size from a forward end along its axis to eventually contact an inwardly-directed valve annulus within the lumen of the fresh valve. The method includes inserting the forward end of the sizing portion into the donor heart valve lumen, and halting the insertion at a predetermined resistance to further insertion. After halting further insertion of the sizing portion into the lumen, the valve annulus size is determined based on the distance that the sizing portion has been inserted. In a preferred embodiment, the sizer further includes a measuring bracket connected thereto having a scale portion spaced from and generally aligned with the sizing portion. The valve size is determined by observing the position of the donor heart valve with respect to the scale portion of the measuring bracket.
In another aspect of the invention, a method of manufacturing prosthetic heart valves is provided. A supply a fresh donor heart valve isolations is provided at a slaughterhouse. The annulus size of the heart valve isolations is measured using a sizer having a sizing portion for insertion within the lumen of the isolation. The method includes selecting a subset of the supply based on the step of measuring, and shipping the subset from slaughterhouse to a valve manufacturing facility. Finally, at least one prosthetic heart valve is fabricated from a donor heart valve isolation selected from the subset.
The present invention also provides an apparatus for sizing a fresh donor heart valve, the fresh valve having a lumen and an inwardly-directed annulus. The apparatus includes a sizer having an axially-extending sizing portion with a forward end adapted to insert within the lumen, the sizing portion having a length of between about 5.08-15.24 cm (2-6 inches). The sizing portion may be conical, and desirably has a taper of between about 2-4 degrees. In one embodiment, the sizing portion is made of a lubricious material, preferably polytetrafluoroethylene. The sizer further may include a measuring bracket connected thereto having a scale portion spaced from and generally aligned with the sizing portion The scale portion provides markings indicating the annulus size of the donor heart valve relative to the position of the donor heart valve on the sizing portion. The markings are desirably supplemented with numerical indicators of valve size, either in terms of valve diameter in millimeters or as non-dimensional numbers in conjunction with a separate chart to correlate the numerical indicators with valve size. The markings may be calibrated for fresh valves from a particular geographic supply source.